Update Sphinx pinned version to get newer python support and remove imghdr requirement

# Required:
------------
[ x] I believe this isn't a duplicate topic
[ x] This report is not related to an adapter

## Select One:

[ x] Build problem
[ ] Incorrect Functionality or bug
[ ] New feature or functionality

## Description

Building docs on Python 3.13 fails due to Sphinx pinned version requiring imghdr which is deprecated in newer python versions as of 3.13 (deprecated but available in 3.12)

## Environment

macOS 14.6 running conda env python 3.13

## Reproduction Steps

```
cd docs
pip install -r requirements
make html
```

fixed by installing python 3.12.

Open docs/requirements and inspect the pinned Sphinx dependency and related requirements. Recreate the issue with Python 3.13 using pip install -r requirements and make html; done means the documentation build succeeds without requiring imghdr.

Written by the indexing model from the issue text.